Osteoporosis is defined as “a skeletal disease characterized by low bone mass and micro-architectural deterioration of bone tissue, with a consequent increase in bone fragility and susceptibility to fracture”. It is a common bone disorder that significantly compromises life quality and shortens life expectancy. Currently, one third of post-menopausal women and one fifth of men over fifty years of age suffer from osteoporosis; the prevalence is expected to rise further as the population ages. Quantitative assessments of the bone strength are required both to detect osteoporosis and monitor treatments. Conventional mechanisms for detecting and monitoring osteoporosis include the measurement of the bone mineral density (BMD), with a lower BMD correlating with lower bone strength and higher fracture risk. However, studies have indicated that approximately half of the post-menopausal women with incident, low energy fractures have a BMD above the World Health Organization threshold definition of osteoporosis. Furthermore, changes in BMD following therapy explain only 4-30% of the observed fracture risk reduction. Thus, a significant amount of research has focused on examining the effect of bone microstructure on bone strength and incidence of osteoporosis; in particular the microstructure of trabecular bone as an indicator of fracture risk has been studied.
Trabecular microstructure has been shown to correlate strongly with bone strength in ex-vivo studies. Non-invasive and in vivo measurements of the trabecular structure, however, are difficult as they require 3D resolutions on the order of 50 μm3. Several techniques have been used to study the microstructure of bone in vivo including radiographs, computed tomography (CT) and magnetic resonance imaging (MRI). These techniques, however, all have limitations in their present implementations. Radiographs provide only a projection of cortical and trabecular microstructure, and therefore only the texture of trabecular bone is resolved. CT requires considerable radiation dose and the resolution of whole body scanners is limited to ˜300 μm3; 80 μm3 resolution is achievable in the periphery, however, peripheral bones are less critical clinically. The significant radiation exposure is particularly problematical for ongoing treatment efficacy trials. Additionally, conventional MRI systems are limited to low resolution (˜300 μm3) due to the poor signal-to-noise ratio known to correspond to such systems and the long time taken to acquire high resolution 3D data.